The invention as hereinafter described relates generally to the field of communications, and is specifically concerned with a method for acquiring burst signal transmissions, particularly spread spectrum transmissions, which contain repeated acquisition codes.
The phrase "spread spectrum" generally refers to methods of radio transmission in which the frequency bandwidth of the transmission greatly exceeds the minimum necessary to communicate the desired information. Numerous types of spread spectrum techniques exist. So-called "direct sequence" systems are those in which the carrier is modulated by a digital code sequence whose bit or "chip" rate is much higher than the information bit rate. "Frequency hopping" systems are those in which the carrier frequency is switched among a plurality of predetermined values. "Chirp modulation" systems sweep the carrier over a wide band during a given pulse interval. These techniques are described in some detail by R.C. Dixon in Spread Spectrum Systems (New York: Wiley and Sons, 1984).
By spreading information over a wide bandwidth relative to the minimum necessary data bandwidth, information may be communicated at relatively low signal-to-noise ratios. This allows signals to be concealed in background noise and provides protection against hostile jamming and unintentional interference.
In some types of direct sequence spread spectrum communication systems, such as radio position determination and message exchange systems, multiple users share the bandwidth simultaneously and the transmissions from any one user occur asynchronously in short bursts, separated by periods of inactivity. The beginning of a burst contains a portion referred to as an acquisition sequence, which is designed to allow a receiver to acquire (i.e., detect the presence of and synchronize to) a burst. Such a portion may be a signal having a high autocorrelation. The remainder of the burst contains the information intended for transmission.
In frequency hopping systems, while one user transmits a burst on one frequency, another user can transmit a burst on a different frequency. In direct sequence systems using pseudo-noise codes, other users appear as background noise. In direct sequence systems, moreover, the amplitude of the signal energy in the communication channel may be less than the amplitude of the noise energy in the channel. That is, signal-to-noise ratios may be much less than one. In such systems, the signal energy is spread over time, and receivers must detect and distinguish signal energy from noise energy.
Many direct sequence spread spectrum systems use surface acoustic wave (SAW) devices at the receiving end. SAW devices convert electrical (voltage) signals into surface waves on a piezoelectric crystal. Because the surface waves of the crystal propagate more slowly than electromagnetic waves, SAW devices may be used to delay a signal in time. SAW devices may be used as code matched filters, that is, filters which are designed to detect a particular code sequence in a signal. Metallic strips which detect the presence of the surface waves may be attached at selected positions on the crystal surface. Such strips can be placed so that they detect the pattern of surface waves which are produced when a signal containing a particular code sequence propagates across the crystal. Matched SAW filters have an output, referred to as a correlation output, which produces an analog signal having a magnitude proportional to the degree of similarity between the input sequence and the sequence for which the filter was designed. When the value of the correlation utput exceeds a threshold value, the system declares an acquisition and other circuitry is employed to demodulate, de-spread and perform further signal processing functions on the received signal.
Technical limits exist on the length of the sequence for which a matched SAW filter may be designed. The maximum sequence length depends in part on the chip rate of the sequence (which determines the time period over which the sequence is transmitted), the propagation speed of the surface wave on the crystal, and the size of the crystal. On the other hand, design limitations on the communication system determine the minimum sequence length which theoretically ensures reliable detection of the burst transmission by the receiver.
Matched SAW filters are designed to detect patterns in signals having a particular center frequency, and the sensitivity of the matched SAW filter decreases for signals whose center frequency is different from the design frequency. That is to say, the magnitude of the correlation output diminishes as the center frequency deviates from the frequency for which the SAW filter was designed. For a given frequency shift, SAW filters designed for long code sequences have a greater loss in sensitivity than SAW filters designed for short code sequences. Toleration of frequency shifts is especially necessary in systems where a receiver must detect the presence of signals from many user terminals which may transmit at frequencies that are slightly shifted from one another (e.g., by virtue of differences among their oscillators, or because a given oscillator may drift over time).
One proposed method of using a matched SAW filter to detect the presence of a long acquisition sequence is to utilize an acquisition sequence which is made up of several short sequences, each having a length for which a single SAW filter can be designed. When a long acquisition sequence made up of repetitions of the short sequence is input to the SAW filter, the SAW filter will produce a series of pulses at the correlation output, one pulse for each repetition of the short sequence. However, the individual pulses may be obscured by noise, because each short sequence is too brief to allow reliable detection in a system which was designed for a long acquisition sequence. Accordingly, the detection pulses from the SAW filter correlation output must be further processed in order to reliably detect the presence of a transmission.
One method that has been used for processing individual detection pulses is to compare each pulse to a threshold value and perform a majority logic operation on the results. That is, if a majority of the received pulses exceed the threshold, then acquisition is declared. Unfortunately, the use of a majority decision suffers from the drawback that it does not utilize the total amount of information in the detected pulses. For example, a detection pulse which is less than the threshold by a small amount is disregarded, just as a pulse which is far below the threshold. Further, a detection pulse which is significantly above the threshold is given no more weight than one which barely exceeds the threshold.
Another method for processing detection pulses that has been used in radar receivers and in decoding algorithms is to quantify the magnitude of each pulse and sum up all the values, either linearly or quadratically. If the sum exceeds a predetermined threshold, then acquisition is declared. However, the use of a summation has the disadvantage that the threshold must be set at a level which is optimized for a particular absolute signal strength, that is, for the particular signal-to-noise ratio of the communication channel. In applications such as radio position determination systems, where transmitters in surface vehicles or aircraft may range over large portions of the earth, the signal-to-noise ratio will vary for each transmitter due to range and local conditions. It is desirable, therefore, to utilize a detection pulse processing scheme which offers good noise rejection for transmissions having variable signal and noise levels.